This invention relates generally to an apparatus for testing the tear strength of elastomers, and, more particularly to an apparatus that is capable of measuring the tear strength or toughness of a crosslinked elastomeric polymer over the useful temperature range of the elastomer.
In the manufacture of products made from elastomeric material, it is important to know as much as possible about the pertinent physical properties of the material in order to knowledgeably control the manufacture of the finished product. For example, it is well recognized that a problem of temporary loss of tensile strength of elastomers can result at high temperatures. More specifically, tensile properties of oil resistant elastomers decrease from approximately 3000 psi at room temperature to 300 to 400 psi at 350.degree. F. When cooled, the elastomers again become tough and strong. The general conclusion reached is that this effect is present with all visco-elastic materials and can generally be predicted from knowing the glass to rubber transistion temperature of the elastomer.
There have also been, however, abnormalities observed such as improved high temperature strength with the utilization of highly reinforcing fillers, high crosslink density and very high molecular weight, linear polymers. In fact, block copolymers have recently been prepared which hold the promise of maintaining strength properties to a higher temperature before severe loss occurs.
By providing an apparatus which is capable of measuring tear propagation in elastomers as a continuous function of temperature it would be possible to set such test equipment at values of temperature which correspond to, for example, systems such as hydraulic systems which incorporate elastomers therein. Thus, it would be possible to select elastomer formulations that were likely to be more durable in such applications.
Unfortunately, through the years the equipment utilized for investigating the properties of elastomers have remained substantially unchanged. Tensile measurements at various temperatures generally require considerable effort and a large number of samples. In many instances, cold and hot tear measurements were conducted which paralleled the tensile loss. This necessitated several specimens at each of several temperatures and therefore required much time and material. Furthermore, prior test equipment had inadequate provisions for temperature control. In addition, in many instances such test equipment was unable to differentiate between soft tough material and those that were of high hardness with poor strength properties. Consequently, there is an urgent need for a simple apparatus than can indicate ultimate tensile strength of elastomers on microspecimens, at various temperatures, and which is capable of measuring such properties independent of occasional flaws in the sample.